applying e-less nickel with or without initial base metal plating?

[pdizon]

From what I understand, I can get the plating thickness for electroless nickel down to .0005 inches. I am planning on plating a machined aluminum part. However, I've been told that I have to initially plate the part with a base metal, such as copper, before I can apply the nickel. Is that true and if so, why? Also, what minimum overall thickness will I have to account for?

Any advice is appreciated.

 

[q1a2d3e4e5r6]

kindly tell me the composition for chromium plating on steels. the current density and the time

 

[kenvlach]

pdizon,
Yes, it is true that Al requires a special pre-plating treatment.* The reason is that the Al surface self-passivates very rapidly: after final cleaning, e.g., in an acidic deoxidizer solution, the rinsing and transferring in air between tanks would leave an Al hydroxide/oxide film which would be difficult to plate upon.

The usual pre-plating is immersion plating in a highly alkaline zincate solution. The pH is so high that any Al hydroxide or oxide dissolves, and a thin layer of Zn plates onto the Al surface in an exchange reaction:
2 Al + 3 Zn⁺² - > 2 Al⁺³ + 3 Zn

There is no need to account for the pre-plating thickness. The zincate layer is extremely small, and most of it dissolves into the EN plating solution. To avoid contamination of main EN plating baths, platers usually have special alkaline EN ‘strike’ baths for an initial EN coating on zincated Al. The parts are then transferred to the main EN bath to get the desired thickness.

A Cu plating isn’t (normally) done before EN plating of Al. It is more common prior to electrolytic plating, especially when buffing or polishing is required before decorative Ni/Cr plating. See ASTM B253 for zincate and zincate+Cu pre-plating solutions and procedures.

For EN on Al, MIL-C-26074E, paragraph 3.3.2 requires a Grade A coating (0.0010” minimum). As your supplier has apparently explained, it is normal to apply at least 0.0005” EN even when not plating to MIL spec in order to get good, pore-free coverage. You might wish to peruse ASTM B656 and ASTM B733 for coating thicknesses based upon service conditions.

*There are EN baths which can plate directly upon Al, but nobody uses them due to cost; the solution gets contaminated by Al.

q1a2d3e4e5r6,

It is better to post new questions separately.

Are you interested in decorative (bright), corrosion-resistant or hard chromium (wear-resistant)? Is this a hardened steel which would require additional processing?

Most chromium plating is done over an initial plating of nickel, or sometimes copper and then nickel for best leveling/polishing and corrosion resistance. I suggest you read ASTM B456 and Federal spec. QQ-C-320B. You can download the latter for free at

http://assist1.daps.dla.mil/quicksearch/.

Hope this helps,
Ken

 

[pdizon]

Hello Ken,

Thanks for the advice and pointing me in the right direction.

Thanks again,
Paul